1. Field of the Invention
The invention relates to automotive engines with multiple cylinders in which a variable displacement characteristic is achieved by activating and deactivating groups of cylinders.
2. Background Art
A multiple-cylinder internal combustion engine in an automotive powertrain usually operates with a four-stroke cycle over a wide speed range and a wide load range. The engine is most efficient when operating with a relatively high load since throttling losses at the air intake are reduced when the engine throttle is at an advanced setting. In order to avoid long-term operation with partial load, it is known design practice to deactivate some of the cylinders of the multiple-cylinder engine so that the active cylinders can be operated at an advanced throttle setting.
One technique for the deactivation of selected cylinders to achieve a variable effective displacement of the engine is to deactivate both the intake and exhaust valves of one or more of the cylinders along with spark and fuel deactivation. This traps air in the deactivated cylinders as the engine operates in its normal four-stroke cycle. The air in the deactivated cylinders is alternately compressed and expanded generally isentropically so that pneumatic spring energy absorbed during the compression stroke is restored as useful work during the expansion stroke. Although friction and thermodynamic losses tend to decrease engine efficiency during operation of the engine with selected cylinders deactivated, that decrease in efficiency is much smaller than the increased efficiency of the active cylinders.
Known variable displacement engine systems that disable both the intake and exhaust valves for the cylinders must, of necessity, carry the cost penalty of the added valve control hardware that is required. An example of an engine with selective cylinder deactivation is described in prior art U.S. Pat. No. 5,934,263. The engine of the ""263 patent includes a multiple-cylinder engine capable of operating on fewer than a full number of cylinders wherein the engine has separate banks of cylinders and an actuator mechanism to equally phase shift the intake and exhaust valves for the cylinders to be deactivated. The valves of the deactivated cylinders can be controlled by dual overhead camshafts or by a single overhead camshaft, but in either case the cam actuators are mechanically linked together. The deactivated cylinders pump exhaust gas back through exhaust valves into a common intake air plenum, which in turn acts as a source of EGR gas for the active cylinders.
Prior art U.S. Pat. No. 5,642,703 discloses a variable displacement engine design in which exhaust valves are deactivated in conjunction with intake cam phasing. This equalizes air flow across the disabled cylinders. It achieves significant cost savings because of the elimination of intake valve deactivators. A cam phase shift of about 60xc2x0 in the retard direction, for example, minimizes the pumping losses of the deactivated cylinders.
Variable displacement engine operation requires carefully sequenced operation of the fuel injectors, spark timing, valve deactivators, electronic throttle actuator, and variable camshaft timing actuators to achieve transitions between cylinder deactivation operating mode and full cylinder activation operating mode. There is a need, however, for providing a smooth transition between the modes in order to minimize disturbances in the effective engine output torque. It is also necessary to carefully control the air-fuel ratio and residual gas levels in the operating cylinders during the transition in order to avoid misfiring during reactivation and to avoid discharge of excessive exhaust gas emissions.
The present invention provides a transition strategy for a control method and a control system to obtain two operating modes by deactivating cylinders through a combination of exhaust valve deactivation and intake cam phasing. It is applicable to an engine having separate banks of cylinders that have different intake cam phasing requirements for the active and inactive cylinders.
A transition from a first operating mode to a second operating mode is achieved by retarding the timing of the intake valve closing event and the intake valve opening event so that the events are approximately equidistant from a top-dead-center position of a piston in a variable displacement engine cylinder. The transition occurs during a so-called torque control region of a time plot, which is followed by a cylinder deactivation region.
The throttle opening is increased during the transition to increase torque of the non-variable displacement cylinders. Camshaft timing is advanced to increase torque for the non-variable displacement cylinders during the transition while retarding camshaft timing and spark timing to reduce torque for the variable displacement cylinders. The exhaust valve, fuel injector, and spark are deactivated for the variable displacement cylinders during a torque control region of the transition. This occurs during one full engine cycle.
Another feature of the invention is the ability of the strategy to control camshaft timing so that the camshaft is phased early in the torque control region before the start of the cylinder deactivation region. This reduces any engine noise that might occur due to rapid expansion of compressed air in the cylinders back into the intake manifold when the intake valve opens.